The present invention generally relates to exposure, and, more particularly, to an exposure method and apparatus for fabricating various types of devices such as semiconductor chips, such as ICs and LSIs, display devices, such as liquid crystal panels, a detection device, such as thin film magnetic heads, and image pickup devices, such as CCDs.
In manufacturing fine semiconductor devices such as a semiconductor memory and a logic circuit in photolithography technology, a reduction projection exposure apparatus has been conventionally employed, which uses a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc., to transfer the circuit pattern.
Recent demands for smaller and finer electronic apparatuses have increasingly called for fine processing to semiconductor devices mounted onto the electronic apparatuses. The critical dimension transferable by the projection exposure apparatus or resolution is inappropriate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. Since the shorter the wavelength is and the higher the NA is, the better the resolution becomes, a wavelength of exposure light is made shorter and the NA of a projection optical system is made higher.
A development of fine processing to circuit patterns has also strictly required a projection optical system to project an image with good image quality. For example, a semiconductor device having a node of 130 nm requires a projected image to restrain a deviation of critical dimensions of a circuit pattern within 10 nm. The demanded image quality of a projected image is met by adjusting residual aberration in a projection optical system to be as small as possible. For this adjustment, some proposed methods optimize a design value and an approach for a projection optical system, improve precision for a fabrication step of the projection optical system, or develop residual aberration adjustment approach and structure, etc. However, a short wavelength of exposure light and a high NA of a projection optical system make it difficult to make the residual aberration small.
Accordingly, the unacceptable image quality degradation due to the residual aberration of a projection optical system has been prevented from affecting manufacture of semiconductor devices by mounting an aberration correction mechanism onto an exposure apparatus, or by adding a fine offset to the NA of a projection optical system and/or the NA of an illumination optical system (although the latter is often replaced with a ratio σ=(NA of the illumination optical system)/(NA of the projection optical system used in an exposure apparatus for semiconductor devices)).
The aberration correction mechanism mounted on the exposure apparatus is able to correct merely wave front aberration of a low order in the residual aberration of the projection optical system. A detailed description will be given of this reason. Wave optics describes an aberrational amount of a projection optical system with dispersed light wave phases in each point on a pupil surface. In other words, it may be defined as distortion of wave front (that is, the same surface with respect to a light wave phase) or wave front aberration. In general, a projection optical system has a circular pupil surface, and thus, the wave front aberration is expressed as Zernike polynomials in Equations 1 and 2 using polar coordinates (r, θ) in the pupil surface. It is general to express an aberration amount in a projection optical system using Zernike coefficients Ci in the exposure apparatus for semiconductor devices:
                    ∑                              C            i                    *                                    R              n              m                        ⁡                          (              r              )                                *                      {                                          cos                ⁢                                                                  ⁢                m                ⁢                                                                  ⁢                θ                            +                              sin                ⁢                                                                  ⁢                m                ⁢                                                                  ⁢                θ                                      }                                              (        1        )                                          R          n          m                =                              ∑                          k              =              0                                                      (                                  n                  -                  m                                )                            /              2                                ⁢                                                    (                                  -                  1                                )                            k                        ⁢                                                                                (                                          n                      -                      k                                        )                                    !                                ⁢                                  r                                      n                    -                                          2                      ⁢                      k                                                                                                                    k                  !                                ⁢                                                      (                                                                                            n                          +                          m                                                2                                            -                      k                                        )                                    !                                ⁢                                                      (                                                                                            n                          -                          m                                                2                                            -                      k                                        )                                    !                                                                                        (        2        )            
Aberration expressed by the Zernike coefficient in which integers n and m are small, or an order of a function with respect to which r is low is called low-order aberration. In the conventional aberration correction mechanism that minutely changes intervals between lenses in the projection optical system or a wavelength of exposure light for aberrational correction may correct the low-order aberration, leaving high-order aberration as residual aberration.
On the other hand, according to a method of adding a fine offset to the NA of a projection optical system and/or the NA of an illumination optical system so as to reduce the image quality degradation, the NA is the only variable parameter and cannot arrest the image quality degradation as required for each semiconductor-device circuit pattern.
It is conceivable to reduce a shape change by adding an offset size or an auxiliary pattern to a circuit pattern on a mask on the assumption of size and shape changes due to the residual aberration in a projected image. However, this requires an appropriate size offset and an auxiliary pattern shape to be determined for each exposure according to a residual aberration in a projection optical system and shape changes in a circuit pattern, making a mask design complex. In addition, additions of a size offset and an auxiliary pattern would increase the mask manufacture cost.
The residual aberration differs among exposure apparatuses. Thus, a mask used for a process that requires the strictest image quality for a circuit pattern may restrain the image quality degradation within a permissible range only in a fixed apparatus, although it spends a long time and requires a high cost for manufacturing the mask. As a consequence, an inefficient operation of an exposure apparatus lowers the productivity or throughput of the semiconductor devices.